objtool/klp: Add test for klp reloc section naming in module objects

klp diff names the intermediate __klp_relocs section after the object the
relocation belongs to, and post-link turns that into
.klp.rela.<object>.<sec>. Name it after the wrong object and the kernel
applies the relocation when the wrong module loads, or never.

The fixture is the first here to honour MODNAME: most hardcode
name=vmlinux, so passing -DMODNAME to them silently does nothing and the
test quietly becomes a vmlinux test.

This tests the behavior of commit 07f14d6af9d7 ("objtool/klp: Fix
cross-module klp relocation section naming").

Assisted-by: Claude:claude-opus-4
Based-on-test-by: Joe Lawrence <joe.lawrence@redhat.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/20260916184351.2720310-29-song@kernel.org

2 files changed, 56 insertions, 0 deletions

diff --git a/tools/objtool/tests/generic/fixtures/cross_module.c b/tools/objtool/tests/generic/fixtures/cross_module.c
new file mode 100644
index 000000000000..c170bde0666f
--- /dev/null
+++ b/tools/objtool/tests/generic/fixtures/cross_module.c
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A function which calls out to another object.  MODNAME selects which object
+ * this one is, so a test can make the caller a module and the callee's owner
+ * something else.
+ */
+
+#ifndef MODNAME
+#define MODNAME "vmlinux"
+#endif
+
+/* klp diff takes the object's module name from .modinfo */
+static const char __modinfo[]
+	__attribute__((section(".modinfo"), used, aligned(1))) = "\0name=" MODNAME;
+
+extern int other_mod_func(int x);
+
+int target(int x)
+{
+#ifdef PATCHED
+	return other_mod_func(x) + 2;
+#else
+	return other_mod_func(x) + 1;
+#endif
+}
diff --git a/tools/objtool/tests/generic/test-module-object.sh b/tools/objtool/tests/generic/test-module-object.sh
new file mode 100755
index 000000000000..95c8402a523a
--- /dev/null
+++ b/tools/objtool/tests/generic/test-module-object.sh
@@ -0,0 +1,31 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# A klp relocation section is named for the object being patched, not for the
+# object which happens to own the symbol being referenced.  Deriving it from
+# the symbol means a cross-module reference lands in a section for an object
+# the patch may not even touch, so the relocation is never applied and the call
+# goes somewhere arbitrary.
+
+. "$(dirname "$0")/../lib.sh"
+
+setup
+build_module_pair cross_module.c klp_testmod
+
+# The fixture has to have built as a module for any of this to mean anything.
+in_sections orig.o | grep -q '\.modinfo' ||
+	fail "fixture has no .modinfo"
+
+# other_mod_func belongs to a different module than the one being patched.
+add_exports other_mod other_mod_func
+run_diff
+
+# Named for the patched object ...
+assert_section __klp_relocs.klp_testmod
+# ... not for the object owning the symbol.
+assert_no_section __klp_relocs.other_mod
+
+run_post_link
+assert_klp_rela klp_testmod .text.target
+
+pass "klp relocation section named for the patched object"